44-6 Rocks on the Move: Long-term Monitoring of Storm Impacts on Coastal Boulder Deposits from San Salvador Island, The Bahamas

Abstract:

Coastal boulder deposits have been monitored since 2012 at two contrasting sites on San Salvador Island, Bahamas: Singer Bar Point (SBP), a low-energy, gently seaward-sloping ramp protected by a reef and lagoon on the north coast; and The Gulf (TG), with 3-5 m cliffs on the high-energy south coast. The initial survey included photographing, describing and GPS locating of 15 large boulders at SBP and 12 at TG, as well as manual profiling of boulder ridges at both sites. 

Smaller size and better roundness of SBP boulders indicate their more frequent movement, yet after Hurricane Sandy passed 80 km west of the island as a Cat. 1-2 storm in Oct. 2012, and Hurricane Joaquin made direct landfall as a strong Cat. 3 storm in Oct. 2015, all 15 monitored boulders remained at or near their original locations. In contrast, these storms moved all but one 4-ton boulder at TG. New boulders as large as 3 m in diameter were emplaced on TG cliffs, and 1-3 ton boulders moved up to 25 m inland with Joaquin’s waves. To aid in post-storm analyses, aerial imagery was first implemented in 2016, and >50 boulders and some cobbles were tagged at each site in 2019 using RFID technology. These advances allowed documentation of modifications at SBP by northerly winter storms in the absence of major hurricanes prior to 2025. During that time, TG cliffs remained mainly unaltered, except for post-Joaquin regrowth of vegetation and some sand transport.    

Continued monitoring produced digital transects for comparison with 2012 manual profiles, and illustrated a 4-5 m landward movement of the seaward edge of boulder deposits at TG and their conversion by Joaquin’s waves from a sharp-crested ridge to a broad boulder field that covered portions of the coastal road. The monitoring also allowed a comparison with Hurricane Melissa, which in Oct. 2025 as a Cat. 1 storm followed a very similar path across the island to that of Joaquin 10-yrs prior. Melissa’s impacts were similar to Joaquin’s in terms of boulder transport, vegetation damage and coastal road inundation, but less extensive. Such long-term monitoring (2012-2026) provides critical information about vulnerability of coastal areas to storms, but its future is uncertain due to increasing complexities of working in The Bahamas.

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